Operation of a phased array antenna at EHF (Extremely High Frequencies) provides important communication system advantages compared to lower frequency operation. The shorter wavelengths at EHF means that a desired antenna gain can be provided by proportionately smaller and lighter apertures. Small size is especially important in applications where conformality of the aperture to a non-planar shape is required. Antenna operation at EHF also allows for more bandwidth at a specified fractional bandwidth, and a less crowded spectrum. Wideband operation is particularly desirable for increasing data rates, reducing the probability of intercept, and negating jamming. However, when driven with appreciable fractional bandwidths and wide scan angles, phased arrays suffer unacceptable losses from frequency-dependent beam pointing, or beam squint. For example, an array of 40.times.40 elements has a gain loss of over 10 dB at a fractional bandwidth of 10 percent. The need for increased bandwidth is unrelenting and will inevitably lead to fractional bandwidth that leads, in turn, to beam squint.
Wide instantaneous bandwidth in high gain arrays will require a true-time-delay (TTD) beam steering approach to avoid the severe scan loss associated with beam squint. Variable time delay (VTD) modules are the key component necessary for implementing TTD beam steering. To practically implement true-time-delay at EHF, the VTD module must be based on a technology that is compatible with low-cost mass producibility, reduced assembly and packaging complexity, as well as being compactly packaged. Switching between different delays must be fast and low loss; delays must be easily and precisely set. Photonics technology offers a practical approach to VTD. The use of photonics brings with it many advantages, including weight and size reduction, EMI immunity, high bandwidth, low loss and negligible dispersion. More important are the advantages of performing signal processing functions such as VTD and phase shifting in the optical domain.
The most common approach to photonic VTD has been an array of electro-optic switches coupled to an array of discrete optical fibers of differing lengths. This approach has serious drawbacks. When the signal is in the very high millimeter wave range, the required delay line can be very short, in the submillimeter to centimeter range. It is not practical to make optical fiber delay lines which are this short, since it is difficult to precisely control fiber length.
Insertion losses are another drawback of fiber optic delay lines. Typical single-mode optical waveguides in Lithium Niobate and glass fiber are between three and nine microns in diameter. To achieve acceptably low coupling loss, alignment between the centers of a waveguide and the optical fiber must be achieved to a tolerance of much less than one micron. This condition is difficult to meet and maintain in any reasonably practical manner.
Fiber optic delay lines are further problematic in that the minimum bending radius is large, usually on the order of a few centimeters, and the required spacing between adjacent fibers is at least 125 microns. These factors combine to pose serious constraints in packing density and increased waveguide bending loss.
Manufacture of a VTD device using optical fiber delay lines presents formidable obstacles. In a typical device, many delay lines must be coupled to waveguides at both ends of the fiber. Each end of each fiber must be individually aligned in a silicon v-groove. This is both time consuming and expensive. The alignment of the fibers in the v-grooves is complicated by the fact that the fiber required for use with typical electro-optic switches is single polarization maintaining fiber. The polarization of the fiber must be maintained in the proper orientation in the v-groove. Failure to do so results not only in unacceptable losses loss but also in crosstalk. It will be readily seen that mass production of fiber optic VTD devices at reasonable cost is nearly impossible as a practical matter.
Another VTD technique uses dispersive fibers and a wavelength-tunable laser source. In addition to the difficulties presented by the use of fibers as discussed above, this technique requires a laser which can overcome challenges of high cost, insufficient wavelength tuning speed, low laser power, laser nonuniformity and wavelength instability. This presents a formidable difficulty. In addition, the multi-picosecond dispersion of the signal across a bandwidth of a few gigahertz can be unacceptable at the 22 ps period of 44 GHz. Finally, packaging of long lengths (hundreds of meters) of dispersive fiber is bulky, tedious, and expensive.
It is apparent that a more easily manufacturable, lower-cost approach to photonic VTD is needed. Likewise, a VTD module with reduced size and weight, which provide fast, low loss switching between different delay lengths that are easily and precisely set, is needed.